With the ignition in the RUN position, and the rear window defogger switch depressed, a class 2 serial data message is sent from the HVAC control module to the body control module (BCM) requesting rear defog operation. The BCM responds by providing ground to the rear defog control signal circuit energizing the R/DEFOG relay. The R/DEFOG relay then provides voltage to the rear defog element supply voltage circuit operating the rear defog system. The HVAC control module illuminates the rear window defogger indicator when a class 2 serial data message is received indicating that the BCM is requesting rear defog operation. The R/DEFOG relay receives battery positive voltage to both the coil and switch input sides from the BATT MAIN 3 fuse in the underhood fuse block. Ground for the rear window defogger is provided at G301.

When you start the engine and press the rear window defogger switch for the first time, the defogger cycle lasts for 15 minutes. Further operation results in 7.5 minute defogger cycles. The defogger cycle resets to 15 minutes when you cycle the ignition to the OFF position and then back to the ON position.

If battery voltage is low, the BCM will disable the rear window defogger.

Schematic Reference

Defogger Schematics

Connector End View Reference

Stationary Windows Connector End Views

Description and Operation

Rear Window Defogger Description and Operation

Electrical Information Reference

Scan Tool Reference

1. Ignition ON, observe the scan tool BCM Rear Defog Relay parameter while pressing the rear window defogger switch. The reading should change between ON and OFF.

⇒	If the reading does not change between the commanded states, replace the HVAC control module.

2. Observe the operation of the rear window defogger indicator while pressing the rear window defogger switch. The indicator should turn ON or OFF each time the rear window defogger switch is pressed.

⇒	If the indicator does not turn ON or OFF with each switch press, replace the HVAC control module.

3. Command the rear window defogger ON and OFF with a scan tool. The rear window defogger should turn ON and OFF when changing between the commanded states.

1. Ignition OFF, disconnect the R/DEFOG relay.
2. Ignition ON, verify a test lamp does not illuminate between terminal J1 and ground.

⇒	If the test lamp illuminates, test circuit J1 for a short to voltage.

3. Verify a test lamp illuminates between terminal J3 and ground.

⇒	If the test lamp does not illuminate, test circuit J3 for a short to ground or an open/high resistance. If the circuit tests normal and the BATT MAIN 3 fuse is open, test or replace the R/DEFOG relay.

4. Verify a test lamp illuminates between terminal K3 and ground.

⇒	If the test lamp does not illuminate, test circuit K3 for a short to ground or an open/high resistance. If the circuit tests normal and the BATT MAIN 3 fuse is open, test circuit J1 for a short to ground.

5. Disconnect the harness connector at the rear window defogger grid.
6. Ignition OFF, test for less than 1 ohm of resistance between the ground circuit terminal A and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

7. Connect the harness connector at the rear window defogger grid.
8. Ignition ON, connect a 40A fused jumper wire between terminal K3 and terminal J1. Verify the rear window defogger grid is activated.

⇒	If the rear window defogger grid does not activate, test circuit J1 for an open/high resistance. If the circuit tests normal, test or replace the rear window defogger grid.

9. Connect a test lamp between terminal J3 and terminal K1.
10. Command the rear window defogger ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test circuit K1 for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

11. If all circuits test normal, test or replace the R/DEFOG relay.

Rear Window Defogger Grid Test

1. Ignition ON, turn on the rear window defogger.
2. Verify that a test lamp illuminates between each rear window defogger grid line and ground. The test lamp should be off when testing at the ground side of the grid and get brighter as the test lamp moves closer to the voltage supply side of the grid. The test lamp should be dim when testing each grid line in the middle.

⇒	If the test lamp remains bright at the middle of the grid line, test for an open between the test point and the ground side of the grid.

⇒	If the test lamp remains off at the middle of the grid line, test for an open between the test point and the voltage supply side of the grid.

Relay Test

1. Ignition OFF, disconnect the R/DEFOG relay.
2. Test for 60-180 ohms of resistance between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 86

•

30 and 87

•

30 and 95

•

85 and 87

⇒	If not the specified value, replace the relay.

4. Install a 5A fused jumper wire between the relay terminal 85 and B+. Install a jumper wire between the relay terminal 86 and ground. Test for less than 2 ohms of resistance between terminals 30 and 87.

⇒	If greater than the specified range, replace the relay.